jQuery.fn.rollover = function(preload) {
    this.filter(':not([src*="_o."])').each(function() {
        var a = this.src, b = this.src.replace(/\.(\w+(\?[^$]*)?)$/, '_o.$1');
        $(this).hover(function() { this.src = b; }, function() { this.src = a; });
        if (preload) {
            var i = new Image;
            i.src = b;
        }
    });
    return this;
};
$(document).ready( function() {
   $('.ro').rollover(true);
});